Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature second stage cryopanel array, usually operating in the range of 4-25 K, is a primary pumping surface. This surface is surrounded by a high temperature radiation shield usually operated in the temperature range of 40-130 K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal cryopanel array positioned between the primary pumping surface and the chamber to be evacuated. This higher temperature, first stage, frontal array serves as a pumping site for high boiling point gases such as water vapor, known as Type I gases.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through the frontal array and into the volume within the radiation shield. Type II gases, such as nitrogen, condense on the second stage array. Type III gases, such as hydrogen, helium and neon, have appreciable vapor pressures at 4K. To capture Type III gases, inner surfaces of the second stage array may be coated with an adsorbent such as activated carbon, zeolite or a molecular sieve. Adsorption is a process whereby gases are physically captured by a material held at cryogenic temperatures and thereby removed from the environment. With the gases thus condensed or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In cryopump systems cooled by closed cycle coolers, the cooler is typically a two stage refrigerator having a cold finger which extends through the radiation shield. The cold end of the second, coldest stage of the refrigerator is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate, a cup or an array of metal baffles arranged around and connected to the second stage heat sink as, for example, in U.S. Pat. Nos. 4,555,907 and 4,494,381, which are incorporated herein by reference. This second stage cryopanel may also support low temperature condensing gas adsorbents such as activated carbon or zeolite as previously stated.
The refrigerator cold finger may extend through the base of a cup-like radiation shield and be concentric with the shield. In other systems, the cold finger extends through the side of the radiation shield. Such a configuration at times better fits the space available for placement of the cryopump.
The radiation shield is connected to a heat sink, or heat station, at the coldest end of the first stage of the refrigerator. This shield surrounds the second stage cryopanel in such a way as to protect it from radiant heat. The frontal array which closes the radiation shield is cooled by the first stage heat sink through the shield or, as disclosed in U.S. Pat. No. 4,356,701, which is incorporated herein by reference, through thermal struts.
Cryopumps need to be regenerated from time to time after large amounts of gas have been collected. Regeneration is a process wherein gases previously captured by the cryopump are released. Regeneration is usually accomplished by allowing the cryopump to return to ambient temperatures and the gases are then removed from the cryopump by means of a secondary pump. Following this release and removal of gas, the cryopump is turned back on and after re-cooling is again capable of removing large amounts of gas from a work chamber.
A figure of merit of cryopumps is the capture probability of hydrogen, the probability that a molecule of hydrogen that reaches the open mouth of the cryopump from outside of the pump will be captured on the second stage of the array. The capture probability directly relates to the speed of the pump for hydrogen, the liters per second captured by the pump. Higher rate pumps of conventional design have a capture probability of hydrogen of 20% or greater.
Various pump designs have been proposed to increase the pumping speed of Type III gases. For example, U.S. Pat. No. 4,718,241, which is incorporated herein by reference, presents a second stage array designed to increase the speed for pumping the non-condensable gases, while at the same time limiting the frequency of regeneration of the system. It accomplishes this by opening up the second stage cryopanel to allow greater accessibility of the noncondensing gases, such as hydrogen, neon, or helium, to the adsorbent material which has been placed on the interior surfaces of the discs of the cryopanel. This allows the noncondensing gases to be adsorbed more quickly, thus increasing the pumping speed for the non-condensables. At the same time, the second stage array was designed so as to assure that all of the gas molecules first strike a surface of the cryopanel which has not been coated with an adsorbent material.
Other pump designs, such as the pump described in U.S. Pat. No. 5,211,022, which is incorporated herein by reference, replace the chevrons or louvers of the first stage with a plate having multiple orifices. The orifices restrict the flow of gases to the second stage compared to the chevrons or louvers. By restricting flow to the inner second stage pumping area, a percentage of inert gases are allowed to remain in the working space to provide a moderate pressure (typically 10−3 Torr or greater) of inert gas for optimal sputtering. However, higher condensing temperature gases, such as water are promptly removed from the environment by condensation on the frontal orifice plate.
The practice of the prior art has been to protect the second stage with chevrons and sputter plates to reduce radiant heat from striking the second stage, to control Type II and III gas flow rates to the second stage, and to prevent Type I, higher boiling point, condensing gases from condensing on the colder surfaces and adsorbent layer. The reduction in radiation and flow rates lowers the temperature of the second stage cryopanel surfaces and the condensed gases on these surfaces as well as the adsorbent. The lower temperature results in an increased gas capture capacity and reduces the frequency of regeneration cycles. The chevrons provide very good radiation shielding as compared to the sputter plates, which contain orifices that provide direct line of sight of the radiant heat to the second stage cryopanel surfaces. However, the current state of the art sputter plates severely restrict Type II and Type III gases to the second stage cryopanels compared to the chevrons, which results in lower pumping speeds for these gases. In some applications, this severe restriction of pumping speed is preferred because a percentage of inert gases are allowed to remain in the working space of the process chamber to provide a moderate pressure of inert gas for optimal sputtering or other processing.